Emergence of tetragonal phase in oxygen-annealed Co-doped La3Ni2O7+s

High-Tc superconductivity concomitant with a structural transition from orthorhombic Amam to tetragonal I4/ mmm phases under high pressure in the Ruddlesden-Popper (RP) phase La3Ni2O7 has recently attracted considerable attention, suggesting an intimate correlation between the tetragonal phase and the emergence of superconductivity. Here, through Co doping followed by oxygen annealing, we successfully obtained tetragonal La3Ni2_ xCoxO7+s polycrystalline bulk samples with the space group I4/mmm. The evolution of crystal structure, oxygen content, electrical transport, density wave transitions and magnetic properties were systematically studied upon Co-doping and oxygen annealing. The oxygen content increased significantly to 7.32, which is essential for stabilizing the tetragonal I4/mmm phase. An insulator-to-metal transition was realized by oxygen annealing, while no signature of superconductivity was observed in the tetragonal phase at ambient pressure. Our work provides a viable method for synthesizing the tetragonal phase, which will facilitate further studies of bilayer nickelate high-Tc superconductors.